Such steam generators, where the heat of vaporization is applied to the feed water by a liquid metal, for example liquid sodium, are used in fast breeder nuclear reactors, the liquid metal being itself heated by another liquid metal cooling the core of the reactor.
The hot liquid sodium is generally led to the upper part of the jacket of the steam generator and flows vertically through the tube bundle.
Each of the tubes of the bundle is connected at one of its ends to a water supply device and at its other end to a collector collecting the steam formed in the tube by heat exchange between the liquid sodium and the feed water flowing in the tube. The water feed device and the steam collector may be constituted by several cylindrical boxes or by toric ducts.
In the case where toric ducts are used, it is known, for example through French Pat. No. 2,449,260, to have a T-connector on each of the tubes, in its terminal part, in the vicinity of its junction point with the toric duct. Such a T-connector enables, in particular, access to be provided to the inside part of the tubes, to carry out checks or measurements.
In such steam generators, it is necessary to limit the consequences of an accident arising from a leakage or a rupture at a tube of the bundle.
In fact, in this case, the water flowing in the tube comes into contact with the liquid sodium and an extremely vigorous chemical reaction ensues. This sodium-water reaction is manifested by two effects: a very short term effect due to the rapid rise in pressure of the hydrogen bubble produced by the reaction, and a second longer term effect arising from the movements of the liquid mass through the pipes of the secondary sodium circuit, under the thrust of the pressurized hydrogen bubble.
The first effect, of very short term, results in the propagation, at the speed of sound, through the steam generator and the pipes of the secondary circuit, of a pressure wave. To provide against this short term effect, it has been proposed to create free surfaces at the upper part of the steam generator or in the tanks situated in the secondary circuit or at different places in the steam generator. Such free surfaces are created by introducing argon into the jacket of the steam generator or into a tank within this steam generator or arranged in the secondary circuit and containing liquid sodium.
During its propagation, the pressure wave is attenuated when it encounters such free surfaces.
There is also arranged at the bottom part of the steam generator, a manifold normally closed by a rupture diaphragm and communicating with one or several rapid drainage tanks for the sodium and the products of the sodium water reaction.
However, the existence of free argon-sodium surfaces results in oscillatory phenomenon in the liquid sodium columns and in the appearance of considerable over-pressures by compression of the covering argon present above these free levels.
In addition, if the sodium-water reaction is prolonged, water continuing to pour into the jacket of the steam generator, the drainage flow rate may become insufficient to avoid a rise in pressure of the steam generator and of the secondary circuit.
Finally, the extremely high flow rates which then traverse the various branches of the secondary circuit create at the terminals of the apparatus located in this circuit extremely high pressure drops and resulting in the destruction of the equipment of the secondary circuit.
Warning signals generated at the time of appearance of the leakage in the tube bundle of the generator actuate rapid decompression valves for the steam as well as stop valves for the feed water delivery to the steam generator.
However, some tens of seconds elapse before the generator is completely isolated and decompressed. This reaction time is excessive, in the case of a leakage corresponding, for example, to the total rupture of a tube, since the amount of water which passes into the liquid sodium is then considerable which results in a very high excess pressure in the secondary circuit and considerable pollution of this circuit.